Color television



April 17, 1956 R. R. LAW

coLoR TELEVISION 3 Sheets-Sheet 1 Filed May 17, 1950 lNVENTOR .Law

ATTORNEY April 17, 1956 R. R. LAW

coLoR TELEVISION 5 Sheets-Sheet 2 Filed May 17. 1950 INVENTCR ]Z.Zaw

MQM

ATTORNEY April 17, 1956 R. R. LAW 2,742,522

COLOR TELEVISION Filed May 17, 1950 3 Sheets-Sheet 3 INVENTOR MQW United States Per@ @me t COLOR TELEVISION 4 Russell. 1RobinLavs", PrincetomN.,1.,:` assignor tok .Radio Corporation of Anjieric'ma corporation of Delaware l Appiicatttm May.y 17,l 195o; seriai'Na. 162,517

' screams.` (ci. 17a- 5.4)

Thisjnyentionrelates to apparatus for improving the col'oitbalance infcolortelevision receivers.; A

- Generally speaking, color televisionsystems operate,l

by transmission of information as to the/intensityl of three dilerent, component colors. ln, most cases the colors selected arey red, green and.. blue.I At the` receiver ployingyphosphors, which emit light of" the same comlonntcolors when struck. by la movingstream ofelectrons. However, due.y to variations in thicknessandlight emission eici'ency, .it often happensthat the` amount of light emitted .by one phosphor for agis/en stimulus or amplitude of a video'signal is different from the amount of light emittedby another. inl response.,v to the. same. stimulus.`

T11liS, th e` ratio, betweenthelights'4 emitted, by the separatevphosphors `is not .thesanre asu1e.ratos.between theKY video signals; representing,the,. component, colors of the image tobe televised-- This disturbs. thecolor' balancein the'- image, reproduced atthereceiver, so, thatthe colors observed are not ,cori-ect.

It i is th,ereforethc object, ,of this, inventionto, `provide an imp rovedmeansv forY balancingu'thearnounttof light f emitted by' the different plftosphorsI man, apparatus; for reproducingncoloredimages. v `Another gurposeoflthe invention `is ,to ,citeercolor bal.- ance in apparatus for, reproducing.,coloredA 'imagesirom transmitted "signals ,in such away as to preventamplitude modulation: from,being ir'1 troduced, f

' Therearebasically two` ways for transntlitting-` the information as to the intensity ofthe.diierent-component colors. In accordance with` Oueknownmethod, tbetransmitte,dsig r 1al sequentially representsthe intensities Ofthe diierent component colors. In accordance with,a.second method;y the ytransmittedsignaler signalscontinuously represents the, intensities of eachroftltetcomponent, colors. '5*0 This inventionV relates toreceivers that 'are adapted to co.- operate withsig nal's that are .Qf'tlieseqUeDtiaItype In Peeuredesf- 17, 1795-6 a4 phosphor.4 which; emitsta particular; 'componente color'- It is then-,onlynecessagyto modulate eachztoythesbeains l Whs-s corresponding Video in'frmationiin; ordem'tcrpro- -ducefa-fulbg/coloredfirnage:r f

, achieved,,by-varyinggthetimmduringewhiclr the ,'beamior j iis less In accordance-,with this;inventions, colon balaneeivis beams of -electronspstrik fhg .dijegnphospho .1 y. extrinsic-f4 itathelighteemittedyby ithe'renfphosphor than,- ,that emitted byfthegreen phosphor;theftelectronsfare Hermittedto strike thered'phosphorlfon longerffperiodsof time -thanthey zare,- permitted, to; strikegthef greenf phosphor; One important f-.featuresofz'this:inventionisf that no-` ,ampliftudemodulation vis1eflectively introducedtiinto'ftherrepros vthese vcolorsmay'be reproduced by cathodray tubes em- Other advantages of; Lthis1inventio11-.\ vllf.:become-apparent 'from a# detailed: consideration of 1 the; drawings :in which lsuchfar.ecei,ver, the transmitted, signal] is. detected ,and "I applied'to the distributor on samplen, The, distributor applies allof the. signalsth'at are representatite,V of'` one component color, for examplgied toatmeans flor-emit'- ting redlight., The other colors are distributed tomeans for reproducingutheir particular colors AThe distributor can thusbe said to sample the received waveandapply ailred samples to one image reproducing devi`c call`l gren samples to another, etc, VIn some CaSeS, frexample, a single4 beam of'electronsisprojected towards a directional screen havingfditerent'color emitting phosphore mounted thereon: The color emitted 'depends upon thed'irection ofjfapproachof thev beam tovth-i`s, s cre en. '[hediiectin ofiapproachlisvari'ed 'as the sequentiall},coloff91h1ation is@ receivedfsothat'l the yredjphfosphor stiguckwhen the -tranemitted*signal 'corresponds totheV intensity? Qfgthe red In other receivers three separate cathode ay-beams are. fiemployede. Each roti .-these bearnsisfdr'ectedttwards apparatus iR" Bew Ori lgure: 3:. illustrates; thef, operation o showrrgine'Figure 2; and# v :f Y* Ftglll `illustrates;tauotheo airangement: whereby'the principles@ of: this :invention fmayf be applied* to=.,asingle bearmltubeffon!reproduoingfcolored fimgefsi heapparatus f rTi'r invention iisg. too described Sin connection with axdotumilt-iplelrisystem.r whereinethe ltransmi-tted'color in# formatiert-ris times divisione multiplexed 'rapidly'withi re-` specttofthe-l efzscanning rater. A description'- offfsuch a 'systerrtunay efounduirt aspnbl'cationientittedg "Synchro: niztieuz EorrCpOlortDotiInterlace; ini The RCAJ Color Tetevisiorn.Systemf` published: bye Radin.- @orporatien of America-and distributed as;:'off;0cttr .berI 1949s l' f -v The, Paltzofe the schematic: arrangement shownv inside oftk `three dotted fboxtz'; of`| Figure-1v is4 directed i towards de rivingttlle videsfgnalssfrom v.the transmittedinformation andere,synchronizingthe des multiplexingattire receiver Withgtlle zd'ot multiplexing: air-ther transmitter and will be describedimdetait:belowV ,v

'A1 special@` typen Aoi:` direct; View single b eamrcolorA re-f producing;tuberithat.'v may` beV` employed j in '-the systemL i's' ilustrated'iimrdetail infFigure 1'; Itfincludes van evacuatedtglass envtelopeat having mounted. 'therein 1an' electron gun Structure .6a` 'lille5 electron gun is-adaptedeto1 project afstreanrioelectrons vtoward afmulti-colorfelement p h'os-u P1101" ScreemfStt Imthes arrangement ShOWnL-the'spotsl of phosphor :thalnemit 3 differenti-colored l lights whenstruck hyf, beamsaiof t electrons,y are arrangedas shownl inf' Figure lfBsf., The-e,spots.f. that-2reproduce red-'lighsare indicated by the letter R, green lightfby'ftlie' letter G,- and"` blue liglt-tbyi ther letter-B. An aperture: 7 in amaslc' 9'l is oricntedsopposite the geometric centerv ofeach of" the gri'rupsrof:phosphorsuV The: screen- 8f is connectedfto'a relatively; high;v positive voltage so. that itfserves 1 the; diuai functionrL ofg collecting@ the electrons and accelerating them.,i i Theintensityfof, the; beam, andfthusi the -intensity ofthecolor reproduced by theicoloredzplrosphorssmotrm ed onthescreen 8, is contro1 1edtby agrictflf AirelectrostaticafocusiugrCylinder 12 of@ standards de: sign serves to focusthe,beamof,electr9ns,emanatinglronr thegun 61'atY thed lig'pertu're 14, and the focusing femasze v t A 4 v a plane transverse to the beam at a point where the" beam in which the beam is bent after-or while'being deflected,

means are provided-for bending the beam in the tube before it is deflected. This change in location of the bending is an improvement in atleast one'respect, namely,

` that it much simpler'to accomplish; Therefore, in between tl1e focusing plates 12 andk the defining aperture 14, the beam is first bent outwardly in an axialA plane, and then inwardly by successive rotating magnetic fields that are always 180 out` of phase with each other. lu this way,.the` direction of approach of the beam to the screen 8 `is continuously changed, although the angle of approach is substantially constant.

The speed of rotation of these fields, and thus the speed of rotation of the axial plane in which the beam is bent and the consequent change in direction of approach of the beam to the screen is determined by the frequency of the voltage wave applied to the primary of a transformer 22. A irst rotating field is established by connecting two pairs of series connected coils 21, 23, 26 and 28 in parallel between one end of the secondary 24 of the transformer 22 and a center tap. Coils 21 and 23 are connected in series with a variable condenser 30 and coils 26 and 28 are connected in series with a variable condenser 32. The condensers 30 and 32 are adjusted so that each pairof field coils forms a parallel resonant circuit with its half of secondary 24 that is sustantially tuned to the frequency of the voltage present in primary 20. The condenser 30 is adjusted so that its parallel resonant circuit is tuned 45 to one side of resonance and the condenser 32 is adjusted so that its parallel resonant circuit is tuned 45 to the other side of resonance so that the magnetic elds are 90 apart. Y pair are mounted on opposite sides of the central axis of the tube and on the outside of the envelope 4, the axis of one pair being at 90 with the other. In this way, a uniformly rotating eld is established at one point in The coils of each is bent away from the axis of the tube. The openings 44 in the diaphragm 42 may, of course, assume any one of numerable configurations. This diaphragm is oriented so that the radii passing through the central points of the openings 44 are substantially parallel to the radii passing through the centers of the spots of phosphors from the geometric center of the groups of phosphors on Figure lB.

lt will be noted that the circumferential lengths of the openings fst in the diaphragm 42 are different. y Assuming that the speed or rotation of the electron beam about the principal axis of the tube is constant, this means that the time that the beam passes through each of the openings will be varied. lf, Afor example, the amount of light emitted by the blue phosphor is greater than the amount emitted by the red and green phosphor, the blue light may be reduced to proper' proportions by reducing me angle between the ends of the appropriate aperture in the diaphragm so that the circumferential length is reduced, as shown.

lt will be noted that when a directional screen of the type described above is employed, there is only one directionof beam approach at which the beam falls on a particular phosphor. As the electron beam rotates about,Y it passes through the aperture 7 and the mask 9 at different angles. Inasmuch as the rotation is synchronized with the rate of rotation of the color information, the beam will strike the phosphor in each group that corresponds to the video modulation appearing on the grid 1.0.

The overall operation of the receiving system shown in Figure l will now be explained in conjunction with the details enclosed in the dotted rectangle 2. Assume, by way of example, that' the wave received and de tected by the standard receiver 46 is comprised of repeated red, blue and green video signals that are time v division multiplexed at a frequency of 3.8 megacycles primed numerals produce the second rotating ield. Be-

cause the voltage supplied these components is supplied by the opposite end of the secondary 24, the second rotating field is 180 out of phase.

Under the influence of the first eld, the electron stream is bent outwardly from the central axis of the tube` and under the influence of the second magnetic eld, the electron stream is bent back toward the central axis. In this way, the electron beam approaches the defining aperture 14 at a constant angle, but it rotates about the central axis of the tube. Upon passing beyond the defining aperture, the beam rotates about the normal path that would be followed by an unbent stream of electrons as it is scanned back and forth over the screen 8. However, the current flowing in the focus coil 16 is so adjusted that the beam is focused at the apertured mask 9, therefore striking the mask 9 at the same point as a beam that isnt bent. Therefore, neglecting the scanning produced by the deflection coils 18, the beam may be said to rotate so as to form a cone the apex of which is at the apertures 7 in the mask 9.

In any sequential system it is necessary to reproduce a given color only when the signals corresponding to that color are available. Therefore, beam interrupting means are provided that block the beam when it approaches the screen at such an angle that it can strike more than one phosphor. This can be accomplished by circuits arranged to key Vthe beam on at appropriate intervals, or it may be accomplished by the insertion of a diaphragm 42 in the path of the beam, such as shown in Figure 1A. The diaphragm '42 is mounted in for each color.

As the scanning proceeds, the phase of these multiplexed signals with respect to a given point in the scanning cycle may be changed.

The phase and frequency of the multiplexing operation at the receiver can be synchronized with that at the transmitter by transmitting a' burst of 3.8 mc. energy of the proper phase on the back porch of the horizontal blanking pulse. Briefly, this burst can be separated from the signals detected by the receiver 46 in the following manner. The horizontal sync pulses supplied by a standard sync separator 4S are differentiated by condenser 5l) and resistor 52. The negative pip thus produced is selected by the diode 54 and inverted in phase inverter 56. The positive pip thus derived keys a flip-op multivibrator 58 that is adjusted to provide a positive pulse of a duration equal to the back porch interval. The applica'- tion of this rectangular pulse to the grid 60 of` a gating tube 62 permits this burst of 3.8 me. energy that is present at the output of receiver 46 and applied to grid 64 of the gating tube 62 to be applied to the grid of an amplifier 66. The amplifier 66 is cathode coupled to a 3.8 mc. oscillator 68. ln this way, the continuously running oseillator 68 is pulled into step with the burst of 3.8 mc. energ and its output is applied to primary 20 of the transformer 22.

The rotating elds discussed above operate at a frcquency of 3.8 mc., and the plane of the beam of electrons is rotated at the same frequency. Inasmuch as the three colors are reproduced in a single rotation of the beam, the video information supplied to the The applicationof this inventionto colored. image restraalt@ producing.v imagesf employingjthree cathode. ray. beams willnow begexplai-neddisconnection with FigpreeZ... n

The yideofsignals .a1;edetected,ina; conventional-manner-.byfaf television` -receivenindicated by,V` thefnumeral 80. They areY applied. to the-control grids 282, 84 andrS-of athreefbeamncathodeffray tube: 88.l Thef grids.. 8,2,-v 84 .kandf86fmay,i forma part ofthreefseparatef eleetronf .gpn s thatare: adaptedto :project a beam.fogelectronsatowards ;a.sereen-.90f The sereenttl4 andwtheemask' 92-thatgis vmounted;` between! it .andy the; gunsrmay be; the.- same@ as that: described; in connection fwith;Figure;V l, l The; sweep ,ceiverwfaree applied/tota :deflectionn yoke 94ay so. as to cause: all V-threegprofzthef beamsgto. lscarmthefscreen'. 9.0; y In this particular:v arrangementithe: electronl beamxcont-rolled by'theugrid 82E approachesthelr :apertures-96: in `the .mask 92;'.at .ang angle suclr` thatiit: always-f strikes thcf'red; spots oiphosphorirnounte'di: on tlrescreen 90.- i Thefbeani'fcomtrolled'by` the grid 84 approachesitheapertures'-.961from `a: ditlerent anglea so; thatzit always; strikes the. greensl phosgridsof .the keyirtgpdeivicesf.-108,;l110A and' .112.

" I peration y' Theoverall operation: otfthisl arrangement'. will be explained? inz-.connectiom with Figure 3.. Assume@ that Vthefsampling` wavek supplied` by the delayfline106'varies aboutiasgroundipotential indieatedbyathe horizontalfline '1541. Aliurtherl assume: that:- the:y upper'wli'ne 156 rcorre- .'spondstozthe potenti'alffof the-cathi'xle:136;?V Thispo'- voltages derived from the conventlonalatelewisioniref phot; and thei'beam;.cont rolled-.-byfv the lgrid-.86.i approaches from ai. still: differentY angle soffthat-"it strikes .thee blue phosphonv f Arr gridf98controlsf the convergence of: theseparate be'amsvso. that=they-cross overatithe-l apertures 9,65- lA gridzf99ffocuses A-eac'h beaml of electrons atf the screen-190. .Th'egrid9f7"ista'screen grid. A y

A-colorsynchronizerf-1001 operatesftoprovide a= sine wave variation-that issynchronized?inl phase andvfeouency witlitlreA sampling operation performed'in the transmitter: color synchronizer may operate inthe fashonfsimil'ar to that describedfin connection'fwith Figure L The frequency thusprovidedis-the;sameaslthe frequencyI 'atfwhichf any given single#v co1`oris repeated'. The output of the color synchronizer is ampliieduinvan amplifier' 1025 Theplatey circuit* off'thi's` ampli-tier vincludesfv a=. variable capacitance 104; the adjustment" of which;v eii'cts` small changes inphase irr` the output ofltlje ampl'iier- 102.'- Any major ,changes inphaserintlie out# ptit-'signal from; the amplier.- 1021' arejeiected-v byj a' 1suitable -d`elay lirhecl.' In this way, the-signals appearing atfthfefjremote endv 'of thetdelay line106 may' be made exactlyinphase; with the sampling*v wave atv the 'trans'- mrtter.Y

r'j'lli'e'* sampling wave appearing jat'v the. output yof' thel dlayj line 106Ky is applied.. to thev control grids"of`tliree separatekeying devices, at phases .thatare'1201degrees apart".` 'Ihese'keying devices may takethe. frtnfofampliii'ers 108 110 and 112,. A"l2-0 delay, line 1`14is connected' between a grid 107i of the .ampliii'e1;.108i an d,a grid109of the amplitier 110. .SimilarlyMa 120 delay Yline ,116' i'siconnected between.` thefgrid 109, and.'.a,.grid

Alll'oftlie ampli'en 1,12. The platesoffthekeyingde:

vices 108,1 110. and.' 112;` are respectively. connectedfto.

plate -load circuits 118, 120 and.12f2. The voltage. ap;

pearing across the load circuit 118 is coupled by a condenser 124.=to. atcathode'1126 of oneiothe electron guns associated Withthe .cathoderray tube. 8.8. The voltage appearing across-"the load" circuit 120 isfcoupled via a condenser-e128.. to. a. cathode.v 13.0.'of; 'another oftthe electron gunsainr the cathoderay.. tube 88; The voltageappearing. across the loadcircuit 122is applied to acathode 132iofr`anoth`er ofthe. electron gunsvia 1a couplingsicondenser 134. Each of the cathodes 126, 130 and 132 is connected to a separate potentiometer. "lfheserpdtentiometersaservefasbackground controls as they establish the bias between the cathode and grid in each of the electron guns.

Cathodes 136, 138 and 140 of the keying devices 108, 110 and 112 are respectively connected to a source of positive potential via variable resistors 142, 144 and 146.

These variable resistors may be separately adjusted so as tential may be varied to'fthat indicated by the. liner-158 -bytfsuitablefladjustmentlofthe variable) res-istor'- 142. .-Aslsume also that1 thescut-.oipotentiali of' thel keying` devieeltl isfrepresentedeby;Bvo1ts,l asindicated bylv the bracket. When"y the= sinusoidali wave' form suppliedi by tl'i'esfline.`r 1625 The importance* of this: fact =is that little Von no aamplitudefmodulation; isucoupled.;` into the cathode ofethes'electronegun: by. thef..keying:.waveffornilty Such amplitude modulation: would .f ofiA course produce distortiorr.inthe:reproducedsmagef v Assume;A now'vrthat :th'evariable resistor; 142:.is: adjusted soil thatztheecathodespotential isanot-f as: great; as 1 inv the case discussedfljusnfabove; Such aapotential is-illustrated by thezline 158th Thereut-:of potential forthe gridi'will, howevergr. be substantiallyi the. same-f amount below this potentiahfand isf'again'indicatedrbyra;bracket Therefore, .ast the..A sampling: Waste;storm;A 1605'riseszabove groundV potential 154, it reaches'rther cut oif'potentialsooner than inthe-case '-previonsly: discussed. Herez again,` cathoder. degeneration,r causes: the plate `current-1 toV follow a lineesnch :as that: indicated 164:- It :isfseenv'that the-conducting period ori theakeyingdeyicev 1081 iselongerf than in thelpreviousrysdescribed example.; 'j

rlherefore, it-carxebes seen ,thatxtheportion of the 1 sam-rpling Waver-during: which -.aiparticularr keying: device.` con.,-

ducts canf be.: established by a suitable` adjustmentuof th'ew variable..` resistors` 142; 144: orsl 146. As, the. con'- ductionfperiodaof th'efkeying devices- 108; 110` and 1112 controls; the time duringi which a: particular *beamI -inthe,

cathode'ray tube- 88t isturned.='on, it can be` seen that thesadjustmentfofthe variable-resistorsv 142; 144VV and 146" eectively controls 1 the colorbalancetherein. The reason fonthis'ri'sY tlfatJ-the amount/of lightproduced is proportional 1to fthe-tirneathat1itstrikesa particular phosphior: p Color balancey can` alsoz be achieved in -a-single/beam tube-suchl as'` shown-tand:l described'l ir'r` connection with Figure 1 by employing circuits instead of the diaphragm 423 Such an arrangement is' illustrated in- Figure 4. The;` dotted-1 rectangle 170 includes those .components includ'ediwithin the1dotted reetangle-170' of FigurefZ. The diiferentl`y-A phased-r outputs of the keyingl devices 108; 1110 and 112 are applied to an adder-1172.I The output of.' the:4 adder 1172'is'vl connected-'to a.: cathode' 174 of; atsingle. beam cathodearayV tube that` is similar to thextube: :4rofti'gurea- 11. on; duri'ngr successive intervals; ottime'.` The relative widths of these intervals are controlled by the biases in thel keying devices 108, and 112 as described in connection with Figure l.

Having described my invention, what is claimed is:

1. An apparatus for producing color balance in a color television receiver that cooperates with transmitted signals representing a plurality of component colors in sequence, comprising in combination a cathode ray tube, a target mountedwithin said tube, a plurality of phos- Thus,.theesinglefbeamv is turned phors that produce diierent colors when stimulated by moving electrons, said phosphors being mounted on said target means for detecting said signals, means providing electrons, means for projecting electrons toward the diterent phosphors in turn and in synchronism with the detected signals, means responsive to said detected signals to modulate said projected electrons, means for interrupting the ow of electrons toward said target in between each separate color signal, said interrupting means adapted to vary therelative duration of the periods during which the electrons impinge upon the different phosphors.

2. An apparatustor controlling color balance in a television receiver that is adapted to cooperate with transmitted signals that in time division multiplex represent the intensities of different component colors in a scanned object, said apparatus comprising in combination means for receiving and detecting said signals, a cathode ray tube having a principal axis, a directional target mounted within said tube, an electron gun for cyclically direct ing a beam of electrons toward said target at different angles, said target being adapted to produce light of a different component color depending on the direction of approach of the electrons in said beam, coils for focusing said beam and deilection coils adapted to cause said beam to scan said target, means for bending the beam of electrons away from the principal axis of said tube before the electrons are deflected and focused by said coils, said bending means being adapted to rotate the plane in which the beam is bent, a diaphragm of beam obstructing material mounted perpendicularlyto the principal axis of the cathode ray tube, apertures within said diaphragm, the orientation of said apertures being such that electrons passing through each one of them impinge on a diterent color producing phosphor, the radial angle subtended by said apertures being dierent so as to alter the relative duration of the periods during which the electrons pass through said apertures.

3. An apparatus for controlling color balance in a television receiver that is adapted tocooperate with transmitted signals that represent the intensities of dierent component colors in a scanned object comprising in combination means for detecting said signals, a plurality of electron guns, a plurality of dilerent phosphors, each phosphor being adapted to emit light of a different cornponent color when struck by moving electrons,.each of said phosphors being mounted so as to be struck only by the electrons projected by a particular gun, means for controlling the intensity of said beams in accordance with said detected signals, a keying device associated with each of said electron guns, means for deriving a voltage wave that is synchronized in phase and frequency with said transmitted signals, connections for applying this voltage wave to said keying devices so as to render them operative sequentially, said keying devices being dif-A ferently biased so that the voltage wave renders the keying devices operative for relatively different periods of time.

4. Apparatus for producing color balance in means for reproducing images in color in response to signals having a lirst component that sequentially represents the intensity of a plurality of colors and a second component that is indicative of the manner in which the colors are represented by the first component comprising in combination a cathode ray tube, a target mounted within said light of a different one of said plurality of colors when struck by a moving electron, means providing electrons,

'means for projecting said electrons toward said groups lto color signals that represent the primary colors cmployed at a given sequential rate comprising in combination cathode ray tube apparatus having a target, intermingled groups of phosphor spots mounted on said target, each group beingadapted to emit light of a different one of said primary colors when struck by moving electrons, a source of control signals having a frequency corresponding to the frequency with which said color signals represent a given primary color, means for formin pulses in response to said control signals, said pulses having different durations, each of said pulses being phased so as to occur when said color signal represents a different primary colormeans providing electrons, means for projecting said electrons toward a phosphor spot that emits light corresponding to the primary color represented by the color signal, means responsive to said color signals for modulating said projected electrons, and means for periodically interrupting said projected electrons and for controlling the time during which the electrons strike a particular phosphor in response to said pulses.

6. In an arrangement for reproducing images in color, apparatus for securing proper color balance comprising in combination a plurality of sources of color signals, each of the signals corresponding to the intensity variations of a dilerent color, a cathode ray tube having a screen, said screen having phosphors that emit light of the different colors represented by the signals, said phosphors being distributed along each line of a raster, means providing electrons, means for sequentially directing said electrons toward each of the phosphors at any given small area in the raster, means responsive to said color signals for varying the number of said electrons directed toward said screen, means for causing the electrons to scan so that the smallV area struck by them moves along each line of the raster, and means for periodically preventing electrons from striking said screen and for controlling the relative times during which electrons are directed toward the diterent phosphors within each of said small areas so that electrons are directed toward one set of said phosphors for longer intervals in each sequence than another set of said phosphors.

References Cited in the tile of this patent UNITED STATES PATENTS 2,406,760 Goldmark Sept. 3, 1946 2,473,276 Chew lune 14, 1949 2,543,772 Goldmark Mar. 5, 1951 2,587,074 Sziklai Feb. 26, 1952 FOREIGN PATENTS 23,777/35 Australia June 4, 1936 

